1. Field of the Invention
The present invention relates to a process for the skeleton isomerization of n-butenes and n-pentenes.
2. Discussion of the Background
The necessity of converting olefins with a linear chain into branched olefins, especially with respect to C4 olefins, is a consequence of the reformulation of benzenes in progress during the last few years. The most recent regulation includes the introduction of a minimum quantity of oxygen. The compound containing oxygen which has proved so far to be the most suitable for this purpose is methylterbutylether (MTBE), which is produced on an industrial scale from methanol and iso-butene. At present the restricting factor in the production of MTBE is the availability of the branched olefin. Processes of skeleton isomerization which allow C4 linear olefins to be used as sources of isobutene have consequently become of extreme interest. In this respect, many catalysts are known for skeleton isomerization, among which alumina (U.S. Pat. No. 4436949, U.S. Pat. No. 4581474 and U.S. Pat. No. 4866211), halogenated alumina (U.S. Pat. No. 4654463, U.S. Pat. No. 4731490, U.S. Pat. No. 4778943, EP 54855, DE 3048693 and EP 71198) and silicized alumina (EP 66485, FR 2657605, DE 3340958, DE 3227676, U.S. Pat. No. 4010590, U.S. Pat. No. 4013589 and U.S. Pat. No. 4038337). To obtain operatively usable conversion values, with catalysts based on alumina, it is necessary to work at low space velocities. With such space velocity values secondary reactions of polymerization and cracking are present. These secondary reactions lower the selectivity of the process and cause the deactivation of the material which must be subjected to reaction cycles and regeneration and must therefore be stable also under the hydrothermal conditions which are created during the regeneration process.
U.S. Pat. No. 2422884 describes a process for the skeleton isomerization of linear olefins which uses as catalyst a suitably prepared alumina on the surface of which is deposited a quantity of boron oxide preferably of between 5.times.10.sup.-4 and 8.6.times.10.sup.-4 g per square meter of alumina. In this patent the optimum concentration of B.sub.2 O.sub.3 is considered to be that which is sufficient to form a monomolecular layer of B.sub.2 O.sub.3 on the surface of the alumina, i.e. a concentration of 6.times.10.sup.-4 gB.sub.2 O.sub.3 /m.sup.2.